Drilling into active faults has become a major scientific endeavor during the last decade and it appears as a most promising
approach to resolve long-standing questions in earthquake and faulting processes. The first boreholes were drilled into the
Nojima Fault following the 1995 Kobe earthquake. Since then, drilling into active faults has begun or been planned in a wide
range of tectonic settings, such as a strike-slip plate boundary (San Andreas Fault, California), a thrust zone in an active
orogenic belt (Chelungpu Fault, Taiwan), a normal fault in an active rift zone (Aigion Fault, Greece), a reactivated Archean
fault (Pretorius Fault, South Africa), and a major subduction thrust (Nankai Thrust, Japan).
These projects have already revealed many details on in-situ stresses, fault-zone structure, fault-rock composition, mechanical
properties, heat flow, and near-field seismicity. Furthermore, most of these projects will continue to serve as observatories
for monitoring fault deformation, fluid pressure and near-field earthquake source processes for a decade or two. Future drilling
projects will focus on near-field observations and long-term monitoring of time-dependent processes and in-situ experimentation
in active fault zones. Collaboration with industry and government will address practical issues pertaining to petroleum and
geothermal energy, radioactive waste disposal, and urban seismic hazards. The outcome of these international efforts in drilling
active faults will revolutionize our understanding of the processes controlling faulting and earthquakes and lead to a stronger
scientific basis for earthquake hazard mitigation.